High purity gold bonding wire for semiconductor device

ABSTRACT

Bonding wire for a semiconductor device contains high purity Au or Au alloy as a base metal and 25-10000 atppm of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au, or contains high purity Au or Au alloy as a base metal and 5-500 atppm of low boiling point element II having a boiling point lower than a melting point of the base metal and insoluble in Au, or contains high purity Au or Au alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au and low boiling point element II having a boiling point lower than the melting point of the base metal and insoluble in Au under the condition of (content of the low boiling point element I)/25+(content of the low boiling point element II)/5≧1≧(content of the low boiling point element I)/10000+(content of the low boiling point element II)/500. The low boiling point elements I and II may be replaced by low boiling point element III soluble in Pd and low boiling point element IV insoluble in Pd, respectively.

BACKGROUND OF THE INVENTION

The present invention relates to a bonding wire for a semiconductor device adapted to be used for connection between a chip electrode of the semiconductor device and an external lead, and more particularly to such a bonding wire suitable for a ball bonding process.

It is known that a chip electrode of a semiconductor device is connected through a bonding wire such as Au wire to an external lead by melting a tip of the Au wire depending from an end of a capillary by means of an electrical torch to form a ball at the tip of the Au wire, pressing the ball against the chip electrode to bond together, then leading the Au wire to the external lead so as to form a loop, bonding the loop to the external lead, and finally cutting the Au wire.

In such a conventional bonding wire for a semiconductor device, however, a neck portion formed immediately above the ball is influenced by heat upon formation of the ball to relax a stress accumulated in the wire. Accordingly, the mechanical strength of the neck portion becomes lower than that of the base wire not influenced by heat. As a result, the neck portion is ruptured, or wire falling or wire sagging is generated during bonding work. Furthermore, in temperature cycle life test of products, a stress is generated by heat expansion and contraction due to repeated temperature change, and the stress is concentrated at the neck portion, causing a problem that the rupture of the neck portion is easily generated.

Meanwhile, in recent years, high-density mounting of LSI has accompanied a marked tendency to provide a multiplicity of pins. In this circumstance, it is demanded to reduce a diameter of the bonding wire and thereby reduce a bonding pitch.

However, as the neck portion of the aforementioned bonding wire is easy to rupture, it is impossible to reduce the diameter of the bonding wire, and the above demand cannot accordingly be met.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to make the strength of the neck portion equal to or more than that of the base wire.

According to a first aspect of the present invention achieve the above object, there is provided a bonding wire for a semiconductor device, containing high purity Au or Au alloy as a base metal and 25-10000 atppm of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au, or there is provided a bonding wire for a semiconductor device, containing high purity Au or Au alloy as a base metal and 5-500 atppm of low boiling point element II having a boiling point lower than a melting point of the base metal and insoluble in Au, or there is provided a bonding wire for a semiconductor device, containing high purity Au or Au alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au and low boiling point element II having a boiling point lower than the melting point of the base metal and insoluble in Au under the condition of (content of the low boiling point element I)/25+(content of the low boiling point element II/5≧1≧(content of the low boiling point element I)/10000+(content of the low boiling point element II)/500.

The aforementioned high purity Au contains 99.99% or more of Au and a residual amount of unavoidable impurities.

The aforementioned Au alloy contains the high purity Au and at least one of Pd, Ag (20 at % or less), Pt (10 at % or less), Rh (2 at % or less), Os, Ru (1 at %), Be, Ca, Ge, Y, La, Mg, Zr, Ga, In, Mo, Re, Cu, Fe (1 atppm-8 at %). The use of this Au alloy improves the mechanical strength of the base metal itself at ordinary temperatures and high temperatures to thereby enable high-speed bonding, and also prevents enlargement of crystal grain in the neck portion upon formation of the ball.

The low boiling point element I having a boiling point lower than the melting point of the base metal and soluble in Au contains at least one of Zn, Cd, Hg, Te, etc., while the low boiling point element II having the boiling point lower than the melting point f the base metal and insoluble in Au contains at least one of P, S, As, Se, Rb, Cs, etc.

According to a second aspect of the present invention achieving the above object, there is provided a bonding wire for a semiconductor device, containing high purity Pd or Pd alloy as a base metal and 25-10000 atppm of low boiling point element III having a boiling point lower than a melting point of the base metal and soluble in Pd, or there is provided a bonding wire for a semiconductor device, containing high purity Pd or Pd alloy as a base metal and 5-500 atppm of low boiling point element IV having a boiling point lower than a melting point of the base metal and insoluble in Pd, or there is provided a bonding wire for a semiconductor device, containing high purity Pd or Pd alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element III having a boiling point lower than a melting point of the base metal and soluble in Pd and low boiling point element IV having a boiling point lower than the melting point of the base metal and insoluble in Pd under the condition of (content of the low boiling point element III)/25+(content of the low boiling point element IV)/5≧1≧(content of the low boiling point element III)/10000+(content of the low boiling point element IV)/500.

The aforementioned high purity Pd contains 99.9% or more of Pd and a residual amount of unavoidable impurities.

The aforementioned Pd alloy contains the high purity Pd and at least one of Au (10 at % or less), Ag (5 at % or less), Pt (20 at % or less), Rh (8 at % or less), Ru, Os, Ir (1 at % or less), Cu, Mo, Fe, Ti (1 atppm-5 at %). The use of this Pd alloy improves mechanical strength of the base metal itself at ordinary temperatures and high temperatures to thereby enable high-speed bonding, and also prevents enlargement of crystal grain in the neck portion upon formation of the ball.

The low boiling point element III having the boiling point lower than the melting point of the base metal and soluble in Pd contains at least one of Zn, Cd, Hg, Li, Be, Mg, Sb, Te, Bi, Yb, etc., while the low boiling point element IV having the boiling point lower than the melting point of the base metal and insoluble in Pd contains at least one of P, S, As, Tl, etc.

The content of each component of the bonding wire for the semiconductor device according to the present invention is limited for the following reasons.

The low boiling point element having the boiling point lower than the melting point of the base metal is vaporized and scattered from the molten ball upon formation thereof. However, the low boiling point element cannot be vaporized from the neck portion, but it tends to be vaporized in the neck portion to generate a stress. Above all, the low boiling point element I soluble in Au is hard to escape from Au. Accordingly, if the content of the low boiling point element I is less than 25 atppm, a satisfactory characteristic cannot be obtained. In contrast, if the content of the low boiling point element I is more than 10000 atppm, embrittlement of the base wire appears to cause difficulty in wire drawing. Moreover, an amount of the element I not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. For the above reasons, the content of the low boiling point element I needs to be set in the range of 25-10000 atppm.

On the other hand, the low boiling point element II insoluble in Au is easy to escape from Au. Accordingly, if the content of the low boiling point element II is less than 5 atppm, a satisfactory characteristic cannot be obtained. In contrast, if the content of the low boiling point element II is more than 500 atppm, embrittlement of the base wire appears to cause difficulty in wire drawing. Moreover, an amount of the element II not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. For the above reasons, the content of the low boiling point element II needs to be set in the range of 5-500 atppm.

Under the condition of (content of the low boiling point element I)/25+(content of said low boiling point element II)/5≧1, if a lower limit of the total content of the low boiling point element I soluble in Au and the low boiling point element II insoluble in Au is less than 5 atppm, a satisfactory characteristic cannot be obtained. Therefore, the lower limit of the total content of both the elements I and II under the above condition needs to be set to 5 atppm.

Further, under the condition of (content of the low boiling point element I)/10000+(content of the low boiling point element II)/500≦1, if an upper limit of the total content of the low boiling point element I soluble in Au and the low boiling point element II insoluble in Au is more than 10000 atppm, embrittlement of the base wire appears to cause difficulty in wire drawing. Moreover, an amount of the element I and II not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. Therefore, the upper limit of the total content of both elements I and II needs to be set to 10000 atppm.

Consequently, the bonding wire of the present invention contains high purity Au or Au alloy as a base metal and 25-10000 atppm of low boiling point element 1 having a boiling point lower than a melting point of the base metal and soluble in Au, or contains high purity Au or Au alloy as a base metal and 5-500 atppm of low boiling point element II having a boiling point lower than a melting point of the base metal and insoluble in Au, or contains high purity Au or Au alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au and low boiling point element II having a boiling point lower than the melting point of the base metal and insoluble in Au under the condition of (content of said low boiling point element I)/25+(content of the low boiling point element II)/5≧1≧(content of the low boiling point element I)/10000+(content of the low boiling point element II)/500. With this constitution, the low boiling point element in the ball is vaporized and scattered upon formation of the ball to thereby prevent gas absorption peculiar to metal and obtain a ball satisfactory for bonding. On the other hand, the low boiling point element in the neck portion cannot be vaporized from the surface thereof, but it tends to be vaporized in the neck portion to generate a stress. Accordingly, a rupture strength of the neck portion after bonding is improved as compared with that of the base wire generating no stress.

The low boiling point element having a boiling point lower than the melting point of the base metal is vaporized and scattered from the molten ball upon formation thereof. However, the low boiling point element cannot be vaporized from the neck portion, but it tends to be vaporized in the neck portion to generate a stress. Above all, the low boiling point element III soluble in Pd is hard to escape from Pd. Accordingly, if the content of the low boiling point element III is less than 25 atppm, a satisfactory characteristic cannot be obtained. In contrast, if the content of the low boiling point element III is more than 10000 atppm, embrittlement of the base wire appears to cause difficulty in wire drawing. Moreover, an amount of the element III not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. For the above reasons, the content of the low boiling point element III needs to be set in the range of 25-10000 atppm.

On the other hand, the low boiling point element IV insoluble in d is easy to escape from Pd. Accordingly, if the content of the low boiling point element IV is less than 6 atppm, a satisfactory characteristic cannot be obtained. In contrast, if the content of the low boiling point element IV is more than 500 atppm, embrittlement of the base wire appears to cause a difficulty of wire drawing. Moreover, an amount of the element IV not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. For the above reasons, the content of the low boiling point element IV needs to be set in the range of 5-500 atppm.

Under the condition of (content of said low boiling point element III)/25+(content of said low boiling point element IV)/5≧1, if a lower limit of the total content of the low boiling point element III soluble in Pd and the low boiling point element IV insoluble in Pd is less than 5 atppm, a satisfactory characteristic cannot be obtained. Therefore, the lower limit of the total content of both the elements III and IV under the above condition needs to be set to 5 atppm.

Further, under the condition of (content of the low boiling point element III)/10000+(content of the low boiling point element IV)/500≦1, if an upper limit of the total content of the low boiling point element III soluble in Pd and the low boiling point element IV insoluble in Pd is more than 10000 atppm, embrittlement of the base wire appears to cause difficulty in wire drawing. Moreover, an amount of the element III and IV not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. Therefore, the upper limit of the total content of both the elements III and IV needs to be set to 10000 atppm.

Consequently, the bonding wire of the present invention contains high purity Pd or Pd alloy as a base metal and 25-10000 atppm of low boiling point element III having a boiling point lower than a melting point of the base metal and soluble in Pd, or contains high purity Pd or Pd alloy as a base metal and 5-500 atppm of low boiling point element IV having a boiling point lower than a melting point of the base metal and insoluble in Pd, or contains high purity Pd or Pd alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element III having a boiling point lower than a melting point of the base metal and soluble in Pd and low boiling point element IV having a boiling point lower than the melting point of the base metal and insoluble in Pd under the condition of (content of the low boiling point element III)/25+(content of the low boiling point element IV)/5≧1 ≧(content of the low boiling point element III)/10000+(content of the low boiling point element IV)/500. With this constitution, the low boiling point element in the ball is vaporized and scattered upon formation of the ball to thereby prevent gas absorption peculiar to metal and obtain a ball satisfactory for bonding. On the other hand, the low boiling point element in the neck portion cannot be vaporized from the surface thereof, but it tends to be vaporized in the neck portion to generate a stress. Accordingly, a rupture strength of the neck portion after bonding is improved as compared with that of the base wire generating no stress.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

There will now be described some preferred embodiments of the present invention.

In a first preferred embodiment employing high purity Au or Au alloy as the base metal, various samples were prepared in the following manner. As the base metal were used high purity Au having a purity of 99.999%, Au alloy containing the high purity Au and 20 at % of Pd, Au alloy containing the high purity Au and 10 atppm of Be, Au alloy containing the high purity Au and 0.5 at % of Mo, and Au alloy containing the high purity Au and 8 at % of Cu. Then, at least one of Zn, Cd, Hg, P and Rb was added to the high purity Au and the Au alloys prepared above, and these mixtures were molten to be cast. The castings thus obtained were then subjected to a grooved roll, during which they were annealed. Then, they were subjected to wire drawing, thereby forming base wires each having a diameter of 30 μm. Thereafter, further stress relieving was sufficiently conducted to finally obtain the samples.

The contents of the additional elements in the samples are shown in Table 1. In Table 1, Nos. 1-54 of the samples correspond to the preferred embodiment, and Nos. 55 and 56 of the samples correspond to comparisons departing from the composition range according to the present invention.

                  TABLE (1)                                                        ______________________________________                                         SAMPLE  ADDITIONAL ELEMENT (at ppm)                                            No.     Zn        Cd      Hg       P    Rb                                     ______________________________________                                         PREFERRED EMBODIMENT                                                            1        20                                                                    2        25                                                                    3        50                                                                    4       500                                                                    5       5000                                                                   6      10000                                                                   7      11000                                                                   8                  20                                                          9                  25                                                         10                  50                                                         11                 500                                                         12                 5000                                                        13                10000                                                        14                11000                                                        15                          20                                                 16                          25                                                 17                          50                                                 18                         500                                                 19                         5000                                                20                        10000                                                21                        11000                                                22                                  3                                          23                                  5                                          24                                  50                                         25                                 500                                         26                                 600                                         27                                       3                                     28                                       5                                     29                                       50                                    30                                      500                                    31                                      600                                    32        10         10                                                        33       100       100                                                         34       1000      1000                                                        35       5000      5000                                                        36       6000      6000                                                        37        10        10      10                                                 38       100       100     100                                                 39       1000      1000    1000                                                40       5000      5000    5000                                                41                                  2    3                                     42                                  5    5                                     43                                  50   50                                    44                                 100  100                                    45                                 250  250                                    46                                 250  300                                    47        10                        2                                          48        10        10              2                                          49                  10              2    2                                     50        10                       250  250                                    51       1000      1000            200  200                                    52       1000      1000     10     200  200                                    53       2000      2000    2000     50   50                                    54       3000      3000    3000     50   50                                    COMPARISON                                                                     55                   Au    (99.99)                                             56                   Cu    (99.9999)                                           ______________________________________                                    

By using the above samples, a pull test was carried out given times (n=40), and measurements were carried out for a pull strength, number of times of rupture of the base wire rather than the neck portion, condition of workability, and existence of chip crack. The result of the measurements for the high purity Au and each Au alloy is shown in Table 2.

                                      TABLE (2)                                    __________________________________________________________________________     Au                          Au + Pd (20 at %)      Au + Be (10 at ppm)                     C                      C                      C                                MODE                   MODE                   MODE                 SAM- PULL   RUP- WORK-      PULL   RUP- WORK-      PULL   RUP-                 PLE  STRENGTH                                                                              TURE ABIL-                                                                               CHIP  STRENGTH                                                                              TURE ABIL-                                                                               CHIP  STRENGTH                                                                              TURE                 NO.  (g)    (n = 40)                                                                            ITY  CRACK (g)    (n = 40)                                                                            ITY  CRACK (g)    (n                   __________________________________________________________________________                                                               = 40)                PREFERRED EMBODIMENT                                                            1   12.0    4   GOOD UNOB- 21.2    6   GOOD UNOB- 12.2    6                                         SERVED                 SERVED                             2   12.1   22   GOOD UNOB- 21.3   21   GOOD UNOB- 12.2   20                                         SERVED                 SERVED                             3   12.2   21   GOOD UNOB- 21.3   25   GOOD UNOB- 12.3   22                                         SERVED                 SERVED                             4   12.8   30   GOOD UNOB- 21.5   28   GOOD UNOB- 12.9   33                                         SERVED                 SERVED                             5   13.0   28   GOOD UNOB- 21.6   30   GOOD UNOB- 13.3   33                                         SERVED                 SERVED                             6   13.4   30   GOOD UNOB- 21.6   33   GOOD UNOB- 13.6   34                                         SERVED                 SERVED                             7   13.6   31   BAD  UNOB- 21.7   32   BAD  OB-   13.7   35                                         SERVED                 SERVED                             8   12.1    7   GOOD UNOB- 21.2    6   GOOD UNOB- 12.2    6                                         SERVED                 SERVED                             9   12.2   21   GOOD UNOB- 21.3   22   GOOD UNOB- 12.2   21                                         SERVED                 SERVED                            10   12.4   23   GOOD UNOB- 21.3   27   GOOD UNOB- 12.4   25                                         SERVED                 SERVED                            11   12.8   35   GOOD UNOB- 21.5   35   GOOD UNOB- 13.0   35                                         SERVED                 SERVED                            12   13.3   36   GOOD UNOB- 21.7   38   GOOD UNOB- 13.5   37                                         SERVED                 SERVED                            13   13.5   34   GOOD UNOB- 21.7   30   GOOD UNOB- 13.7   36                                         SERVED                 SERVED                            14   13.6   35   BAD  UNOB- 21.8   31   BAD  OB-   13.7   35                                         SERVED                 SERVED                            15   12.0    9   GOOD UNOB- 21.1    8   GOOD UNOB- 12.3    8                                         SERVED                 SERVED                            16   12.1   26   GOOD UNOB- 21.2   24   GOOD UNOB- 12.4   27                                         SERVED                 SERVED                            17   12.8   33   GOOD UNOB- 21.2   36   GOOD UNOB- 12.5   34                                         SERVED                 SERVED                            18   13.4   40   GOOD UNOB- 21.5   38   GOOD UNOB- 13.1   39                                         SERVED                 SERVED                            19   13.5   38   GOOD UNOB- 21.7   36   GOOD UNOB- 13.4   39                                         SERVED                 SERVED                            20   13.7   35   GOOD UNOB- 21.7   34   GOOD UNOB- 13.7   36                                         SERVED                 SERVED                            21   13.7   36   BAD  UNOB- 21.8   37   BAD  OB-   13.7   35                                         SERVED                 SERVED                            22   12.0   13   GOOD UNOB- 21.0   12   GOOD UNOB- 12.2   12                                         SERVED                 SERVED                            23   12.0   22   GOOD UNOB- 21.0   20   GOOD UNOB- 12.2   24                                         SERVED                 SERVED                            24   12.2   38   GOOD UNOB- 21.1   35   GOOD UNOB- 12.7   36                                         SERVED                 SERVED                            25   12.5   37   GOOD UNOB- 21.4   37   GOOD UNOB- 13.3   40                                         SERVED                 SERVED                            26   12.7   38   BAD  UNOB- 21.5   34   BAD  UNOB- 13.4   38                                         SERVED                 SERVED                            27   12.0   12   GOOD UNOB- 21.1   12   GOOD UNOB- 12.3   11                                         SERVED                 SERVED                            28   12.0   22   GOOD UNOB- 21.1   25   GOOD UNOB- 12.3   21                                         SERVED                 SERVED                            29   12.3   35   GOOD UNOB- 21.2   36   GOOD UNOB- 13.0   28                                         SERVED                 SERVED                            30   12.4   37   GOOD UNOB- 21.8   38   GOOD UNOB- 13.5   29                                         SERVED                 SERVED                            31   12.5   38   BAD  UNOB- 21.8   38   BAD  UNOB- 13.5   30                                         SERVED                 SERVED                            32   12.1    7   GOOD UNOB- 21.2    7   GOOD UNOB- 12.7    6                                         SERVED                 SERVED                            33   12.3   33   GOOD UNOB- 21.4   35   GOOD UNOB- 12.9   37                                         SERVED                 SERVED                            34   13.1   37   GOOD UNOB- 21.6   37   GOOD UNOB- 13.4   38                                         SERVED                 SERVED                            35   13.5   35   GOOD UNOB- 21.7   33   GOOD UNOB- 13.7   35                                         SERVED                 SERVED                            36   13.6   36   BAD  UNOB- 21.8   34   BAD  OB-   13.7   35                                         SERVED                 SERVED                            37   12.5   28   GOOD UNOB- 21.2   30   GOOD UNOB- 12.3   25                                         SERVED                 SERVED                            38   12.7   36   GOOD UNOB- 21.3   34   GOOD UNOB- 12.6   28                                         SERVED                 SERVED                            39   13.0   30   GOOD UNOB- 21.8   29   GOOD UNOB- 13.3   30                                         SERVED                 SERVED                            40   13.8   35   BAD  UNOB- 21.8   30   BAD  OB-   13.8   32                                         SERVED                 SERVED                            41   12.0   23   GOOD UNOB- 21.1   27   GOOD UNOB- 12.2   25                                         SERVED                 SERVED                            42   12.1   29   GOOD UNOB- 21.1   31   GOOD UNOB- 12.2   27                                         SERVED                 SERVED                            43   12.4   28   GOOD UNOB- 21.4   35   GOOD UNOB- 12.6   34                                         SERVED                 SERVED                            44   12.4   36   GOOD UNOB- 21.4   38   GOOD UNOB- 12.9   39                                         SERVED                 SERVED                            45   12.5   37   GOOD UNOB- 21.8   36   GOOD UNOB- 13.5   33                                         SERVED                 SERVED                            46   12.4   35   BAD  UNOB- 21.8   34   BAD  UNOB- 13.5   33                                         SERVED                 SERVED                            47   12.0   12   GOOD UNOB- 21.1   12   GOOD UNOB- 12.2   12                                         SERVED                 SERVED                            48   12.1   23   GOOD UNOB- 21.2   21   GOOD UNOB- 12.3   25                                         SERVED                 SERVED                            49   12.1   27   GOOD UNOB- 21.2   25   GOOD UNOB- 12.3   29                                         SERVED                 SERVED                            50   12.5   36   BAD  UNOB- 21.6   33   BAD  UNOB- 13.5   35                                         SERVED                 SERVED                            51   13.2   36   GOOD UNOB- 21.6   38   GOOD UNOB- 13.5   39                                         SERVED                 SERVED                            52   13.3   35   BAD  UNOB- 21.6   34   BAD  UNOB- 13.6   36                                         SERVED                 SERVED                            53   13.5   28   GOOD UNOB- 21.8   35   GOOD UNOB- 13.6   34                                         SERVED                 SERVED                            54   13.7   29   BAD  UNOB- 21.8   34   BAD  OB-   13.7   37                                         SERVED                 SERVED                            COMPARISON                                                                          PULL STRENGTH (g)                                                         55   12                                                                        56   21                                                                        __________________________________________________________________________                     Au + Mo (0.5 at %)      Au + Cu (8 at %)                                               C                      C                               Au + Be (10 at ppm)     MODE                   MODE                            SAM- WORK-      PULL    RUP- WORK-      PULL   RUP- WORK-                      PLE  ABIL-                                                                               CHIP  STRENGTH                                                                               TURE ABIL-                                                                               CHIP  STRENGTH                                                                              TURE ABIL-                                                                               CHIP                  NO.  ITY  CRACK (g)     (n = 40)                                                                            ITY  CRACK (g)    (n = 40)                                                                            ITY  CRACK                 __________________________________________________________________________     PREFERRED EMBODIMENT                                                            1   GOOD UNOB- 13.0     7   GOOD UNOB- 21.9    5   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 2   GOOD UNOB- 13.1    22   GOOD UNOB- 22.0   21   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 3   GOOD UNOB- 13.1    24   GOOD UNOB- 22.0   26   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 4   GOOD UNOB- 13.5    27   GOOD UNOB- 22.2   32   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 5   GOOD UNOB- 13.6    29   GOOD UNOB- 22.3   36   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 6   GOOD UNOB- 13.8    29   GOOD UNOB- 22.4   35   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 7   BAD  UNOB- 13.8    32   BAD  OB-   22.5   33   BAD  OB-                             SERVED                  SERVED                 SERVED                 8   GOOD UNOB- 13.1     8   GOOD UNOB- 22.0    4   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 9   GOOD UNOB- 13.2    23   GOOD UNOB- 22.1   22   GOOD UNOB-                           SERVED                  SERVED                 SERVED                10   GOOD UNOB- 13.2    26   GOOD UNOB- 22.1   23   GOOD UNOB-                           SERVED                  SERVED                 SERVED                11   GOOD UNOB- 13.3    30   GOOD UNOB- 22.2   34   GOOD UNOB-                           SERVED                  SERVED                 SERVED                12   GOOD UNOB- 13.5    33   GOOD UNOB- 22.4   36   GOOD UNOB-                           SERVED                  SERVED                 SERVED                13   GOOD UNOB- 13.6    31   GOOD UNOB- 22.4   36   GOOD UNOB-                           SERVED                  SERVED                 SERVED                14   BAD  UNOB- 13.8    34   BAD  OB-   22.5   32   BAD  OB-                             SERVED                  SERVED                 SERVED                15   GOOD UNOB- 13.3    11   GOOD UNOB- 22.0    9   GOOD UNOB-                           SERVED                  SERVED                 SERVED                16   GOOD UNOB- 13.3    29   GOOD UNOB- 22.0   27   GOOD UNOB-                           SERVED                  SERVED                 SERVED                17   GOOD UNOB- 13.4    30   GOOD UNOB- 22.1   31   GOOD UNOB-                           SERVED                  SERVED                 SERVED                18   GOOD UNOB- 13.5    35   GOOD UNOB- 22.3   38   GOOD UNOB-                           SERVED                  SERVED                 SERVED                19   GOOD UNOB- 13.7    36   GOOD UNOB- 22.4   36   GOOD UNOB-                           SERVED                  SERVED                 SERVED                20   GOOD UNOB- 13.7    35   GOOD UNOB- 22.5   37   GOOD UNOB-                           SERVED                  SERVED                 SERVED                21   BAD  OB-   13.8    35   BAD  OB-   22.5   33   BAD  OB-                             SERVED                  SERVED                 SERVED                22   GOOD UNOB- 13.2    14   GOOD UNOB- 22.1   10   GOOD UNOB-                           SERVED                  SERVED                 SERVED                23   GOOD UNOB- 13.2    25   GOOD UNOB- 22.1   33   GOOD UNOB-                           SERVED                  SERVED                 SERVED                24   GOOD UNOB- 13.5    39   GOOD UNOB- 22.2   40   GOOD UNOB-                           SERVED                  SERVED                 SERVED                25   GOOD UNOB- 13.7    39   GOOD UNOB- 22.5   39   GOOD UNOB-                           SERVED                  SERVED                 SERVED                26   BAD  UNOB- 13.7    38   BAD  UNOB- 22.5   39   BAD  UNOB-                           SERVED                  SERVED                 SERVED                27   GOOD UNOB- 13.1    13   GOOD UNOB- 22.1    9   GOOD UNOB-                           SERVED                  SERVED                 SERVED                28   GOOD UNOB- 13.1    23   GOOD UNOB- 22.1   30   GOOD UNOB-                           SERVED                  SERVED                 SERVED                29   GOOD UNOB- 13.2    37   GOOD UNOB- 22.2   38   GOOD UNOB-                           SERVED                  SERVED                 SERVED                30   GOOD UNOB- 13.6    36   GOOD UNOB- 22.4   35   GOOD UNOB-                           SERVED                  SERVED                 SERVED                31   BAD  UNOB- 13.6    36   BAD  UNOB- 22.4   35   BAD  UNOB-                           SERVED                  SERVED                 SERVED                32   GOOD UNOB- 13.1     7   GOOD UNOB- 22.1    5   GOOD UNOB-                           SERVED                  SERVED                 SERVED                33   GOOD UNOB- 13.2    33   GOOD UNOB- 22.1   34   GOOD UNOB-                           SERVED                  SERVED                 SERVED                34   GOOD UNOB- 13.4    35   GOOD UNOB- 22.3   35   GOOD UNOB-                           SERVED                  SERVED                 SERVED                35   GOOD UNOB- 13.6    22   GOOD UNOB- 22.4   36   GOOD UNOB-                           SERVED                  SERVED                 SERVED                36   BAD  UNOB- 13.8    36   BAD  OB-   22.5   34   BAD  OB-                             SERVED                  SERVED                 SERVED                37   GOOD UNOB- 13.4    30   GOOD UNOB- 22.2   23   GOOD UNOB-                           SERVED                  SERVED                 SERVED                38   GOOD UNOB- 13.5    34   GOOD UNOB- 22.3   30   GOOD UNOB-                           SERVED                  SERVED                 SERVED                39   GOOD UNOB- 13.7    37   GOOD UNOB- 22.3   33   GOOD UNOB-                           SERVED                  SERVED                 SERVED                40   BAD  UNOB- 13.7    35   BAD  OB-   22.3   31   BAD  OB-                             SERVED                  SERVED                 SERVED                41   GOOD UNOB- 13.1    27   GOOD UNOB- 22.1   30   GOOD UNOB-                           SERVED                  SERVED                 SERVED                42   GOOD UNOB- 13.3    28   GOOD UNOB- 22.2   34   GOOD UNOB-                           SERVED                  SERVED                 SERVED                43   GOOD UNOB- 13.4    38   GOOD UNOB- 22.3   38   GOOD UNOB-                           SERVED                  SERVED                 SERVED                44   GOOD UNOB- 13.2    36   GOOD UNOB- 22.1   35   GOOD UNOB-                           SERVED                  SERVED                 SERVED                45   GOOD UNOB- 13.6    35   GOOD UNOB- 22.4   33   GOOD UNOB-                           SERVED                  SERVED                 SERVED                46   BAD  UNOB- 13.6    37   BAD  UNOB- 22.4   35   BAD  UNOB-                           SERVED                  SERVED                 SERVED                47   GOOD UNOB- 13.1    14   GOOD UNOB- 22.1   10   GOOD UNOB-                           SERVED                  SERVED                 SERVED                48   GOOD UNOB- 13.0    27   GOOD UNOB- 22.0   33   GOOD UNOB-                           SERVED                  SERVED                 SERVED                49   GOOD UNOB- 13.1    31   GOOD UNOB- 22.1   32   GOOD UNOB-                           SERVED                  SERVED                 SERVED                50   BAD  UNOB- 13.4    37   BAD  UNOB- 22.3   32   BAD  UNOB-                           SERVED                  SERVED                 SERVED                51   GOOD UNOB- 13.6    36   GOOD UNOB- 22.4   35   GOOD UNOB-                           SERVED                  SERVED                 SERVED                52   BAD  UNOB- 13.5    33   BAD  UNOB- 22.3   36   BAD  UNOB-                           SERVED                  SERVED                 SERVED                53   GOOD UNOB- 13.7    38   GOOD UNOB- 22.4   38   GOOD UNOB-                           SERVED                  SERVED                 SERVED                54   BAD  UNOB- 13.8    35   BAD  OB-   22.5   38   BAD  OB-                             SERVED                  SERVED                 SERVED                COMPARISON                                                                          C MODE RUPTURE (n = 40)                                                   55   2                                                                         56   2                                                                         __________________________________________________________________________

As apparent from Table 2, it is appreciated that the number of times of C mode rupture in the pull test by using the samples according to the present invention is larger than that by using the samples in the comparisons departing from the composition range according to the present invention, and that the neck portion is stronger than the other portion of the base wire. Accordingly, it is understood that the above-mentioned composition range according to the present invention is optimum.

In a second preferred embodiment employing high purity Pd or Pd alloy as the base metal, various samples were prepared in the following manner. As the base metal were used high purity Pd having a purity of 99.95% Pd alloy containing the high purity Pd and 10 at % of Au, Pd alloy containing the high purity Pd and 5 atppm of Ag, Pd alloy containing the high purity Pd and 0.5 at % of Mo, and Pd alloy containing the high purity Pd and 6 at % of Cu. Then, at least one of Zn, Cd, Hg, P and S was added to the high purity Pd and the Pd alloys prepared above, and these mixtures were molten to be cast. The castings thus obtained were then subjected to a grooved roil, during which they were annealed. Then, they were subjected to wire drawing, thereby forming base wires each having a diameter of 30 μm. Thereafter, further stress relieving was sufficiently conducted to finally obtain the samples.

The contents of the additional elements in the samples are shown in Table 3. In Table 3, Nos. 1-64 of the samples correspond to the preferred embodiment, and Nos. 55 and 56 of the samples correspond to comparisons departing from the composition range according to the present invention.

                  TABLE (3)                                                        ______________________________________                                         SAMPLE  ADDITIONAL ELEMENT (at ppm)                                            No.     Zn        Cd      Hg       P    S                                      ______________________________________                                         PREFERRED EMBODIMENT                                                            1        20                                                                    2        25                                                                    3        50                                                                    4       500                                                                    5       5000                                                                   6      10000                                                                   7      11000                                                                   8                  20                                                          9                  25                                                         10                  50                                                         11                 500                                                         12                 5000                                                        13                10000                                                        14                11000                                                        15                          20                                                 16                          25                                                 17                          50                                                 18                         500                                                 19                         5000                                                20                        10000                                                21                        11000                                                22                                  3                                          23                                  5                                          24                                  50                                         25                                 500                                         26                                 600                                         27                                       3                                     28                                       5                                     29                                       50                                    30                                      500                                    31                                      600                                    32        10         10                                                        33       100       100                                                         34       1000      1000                                                        35       5000      5000                                                        36       6000      6000                                                        37        10        10      10                                                 38       100       100     100                                                 39       1000      1000    1000                                                40       5000      5000    5000                                                41                                  2    3                                     42                                  5    5                                     43                                  50   50                                    44                                 100  100                                    45                                 250  250                                    46                                 250  300                                    47        10                        2                                          48        10        10              2                                          49                  10              2    2                                     50        10                       250  250                                    51       1000      1000            200  200                                    52       1000      1000     10     200  200                                    53       2000      2000    2000     50   50                                    54       3000      3000    3000     50   50                                    COMPARISON                                                                     55                   Au    (99.99)                                             56                   Cu    (99.9999)                                           ______________________________________                                    

By using the above samples, a pull test was carried out given times (n=40), and measurements were carried out for a pull strength, number of times of rupture of the base wire rather than the neck portion, condition of workability, and existence of chip crack. The result of the measurements for the high purity Pd and each Pd alloy is shown in Table 4.

                                      TABLE (4)                                    __________________________________________________________________________     Pd                          Pd + Au (10 at %)      Pd + Ag (5 at %)                        C                      C                      C                                MODE                   MODE                   MODE                 SAM- PULL   RUP- WORK-      PULL   RUP- WORK-      PULL   RUP-                 PLE  STRENGTH                                                                              TURE ABIL-                                                                               CHIP  STRENGTH                                                                              TURE ABIL-                                                                               CHIP  STRENGTH                                                                              TURE                 NO.  (g)    (n = 40)                                                                            ITY  CRACK (g)    (n = 40)                                                                            ITY  CRACK (g)    (n                   __________________________________________________________________________                                                               = 40)                PREFERRED EMBODIMENT                                                            1   19.0    4   GOOD UNOB- 21.3    7   GOOD UNOB- 21.5    6                                         SERVED                 SERVED                             2   19.0   22   GOOD UNOB- 21.3   25   GOOD UNOB- 21.5   22                                         SERVED                 SERVED                             3   19.2   27   GOOD UNOB- 21.4   29   GOOD UNOB- 21.5   30                                         SERVED                 SERVED                             4   19.7   35   GOOD UNOB- 21.5   32   GOOD UNOB- 21.6   33                                         SERVED                 SERVED                             5   19.9   33   GOOD UNOB- 21.6   31   GOOD UNOB- 21.7   34                                         SERVED                 SERVED                             6   20.1   35   GOOD UNOB- 21.7   34   GOOD UNOB- 21.7   34                                         SERVED                 SERVED                             7   20.2   30   BAD  UNOB- 21.7   35   BAD  OB-   21.8   32                                         SERVED                 SERVED                             8   19.0    5   GOOD UNOB- 21.3    7   GOOD UNOB- 21.5    6                                         SERVED                 SERVED                             9   19.1   21   GOOD UNOB- 21.3   23   GOOD UNOB- 21.5   21                                         SERVED                 SERVED                            10   19.2   27   GOOD UNOB- 21.4   25   GOOD UNOB- 21.6   27                                         SERVED                 SERVED                            11   19.7   36   GOOD UNOB- 21.5   32   GOOD UNOB- 21.6   33                                         SERVED                 SERVED                            12   20.0   35   GOOD UNOB- 21.5   35   GOOD UNOB- 21.7   37                                         SERVED                 SERVED                            13   20.1   32   GOOD UNOB- 21.6   33   GOOD UNOB- 21.7   34                                         SERVED                 SERVED                            14   20.3   31   BAD  UNOB- 21.6   35   BAD  OB-   21.8   36                                         SERVED                 SERVED                            15   19.1    4   GOOD UNOB- 21.2   7    GOOD UNOB- 21.4    5                                         SERVED                 SERVED                            16   19.1   23   GOOD UNOB- 21.3   27   GOOD UNOB- 21.5   22                                         SERVED                 SERVED                            17   19.2   30   GOOD UNOB- 21.4   33   GOOD UNOB- 21.6   28                                         SERVED                 SERVED                            18   19.5   38   GOOD UNOB- 21.6   39   GOOD UNOB- 21.7   36                                         SERVED                 SERVED                            19   19.9   37   GOOD UNOB- 21.6   39   GOOD UNOB- 21.7   37                                         SERVED                 SERVED                            20   20.2   38   GOOD UNOB- 21.7   36   GOOD UNOB- 21.7   34                                         SERVED                 SERVED                            21   20.3   33   BAD  UNOB- 21.7   35   BAD  OB-   21.8   31                                         SERVED                 SERVED                            22   19.0   12   GOOD UNOB- 21.3   10   GOOD UNOB- 21.5    9                                         SERVED                 SERVED                            23   19.1   21   GOOD UNOB- 21.3   21   GOOD UNOB- 21.5   20                                         SERVED                 SERVED                            24   19.3   33   GOOD UNOB- 21.4   29   GOOD UNOB- 21.5   31                                         SERVED                 SERVED                            25   19.6   37   GOOD UNOB- 21.5   35   GOOD UNOB- 21.5   36                                         SERVED                 SERVED                            26   19.7   35   BAD  UNOB- 21.6   35   BAD  UNOB- 21.6   33                                         SERVED                 SERVED                            27   19.0   10   GOOD UNOB- 21.3   10   GOOD UNOB- 21.5   12                                         SERVED                 SERVED                            28   19.2   22   GOOD UNOB- 21.3   20   GOOD UNOB- 21.5   21                                         SERVED                 SERVED                            29   19.3   30   GOOD UNOB- 21.3   27   GOOD UNOB- 21.5   28                                         SERVED                 SERVED                            30   19.6   36   GOOD UNOB- 21.4   32   GOOD UNOB- 21.6   31                                         SERVED                 SERVED                            31   19.7   37   BAD  UNOB- 21.5   36   BAD  UNOB- 21.6   35                                         SERVED                 SERVED                            32   19.1    6   GOOD UNOB- 21.3    3   GOOD UNOB- 21.5    3                                         SERVED                 SERVED                            33   19.2   22   GOOD UNOB- 21.4   23   GOOD UNOB- 21.5   24                                         SERVED                 SERVED                            34   19.8   28   GOOD UNOB- 21.5   29   GOOD UNOB- 21.6   30                                         SERVED                 SERVED                            35   20.1   35   GOOD UNOB- 21.6   32   GOOD UNOB- 21.7   33                                         SERVED                 SERVED                            36   20.3   36   BAD  UNOB- 21.6   34   BAD  OB-   21.8   35                                         SERVED                 SERVED                            37   19.2   27   GOOD UNOB- 21.4   28   GOOD UNOB- 21.5   30                                         SERVED                 SERVED                            38   19.5   38   GOOD UNOB- 21.5   37   GOOD UNOB- 21.6   35                                         SERVED                 SERVED                            39   19.8   37   GOOD UNOB- 21.5   37   GOOD UNOB- 21.6   36                                         SERVED                 SERVED                            40   20.4   33   BAD  UNOB- 21.6   36   BAD  OB-   21.8   33                                         SERVED                 SERVED                            41   19.1   23   GOOD UNOB- 21.3   21   GOOD UNOB- 21.5   22                                         SERVED                 SERVED                            42   19.3   29   GOOD UNOB- 21.3   32   GOOD UNOB- 21.5   28                                         SERVED                 SERVED                            43   19.4   38   GOOD UNOB- 21.3   37   GOOD UNOB- 21.5   36                                         SERVED                 SERVED                            44   19.5   37   GOOD UNOB- 21.3   35   GOOD UNOB- 21.5   38                                         SERVED                 SERVED                            45   19.6   36   GOOD UNOB- 21.5   34   GOOD UNOB- 21.5   37                                         SERVED                 SERVED                            46   19.7   36   BAD  UNOB- 21.5   35   BAD  UNOB- 21.5   36                                         SERVED                 SERVED                            47   19.1    6   GOOD UNOB- 21.3    7   GOOD UNOB- 21.5    7                                         SERVED                 SERVED                            48   19.2   20   GOOD UNOB- 21.3   22   GOOD UNOB- 21.5   21                                         SERVED                 SERVED                            49   19.2   24   GOOD UNOB- 21.4   26   GOOD UNOB- 21.5   25                                         SERVED                 SERVED                            50   19.6   37   BAD  UNOB- 21.5   37   BAD  UNOB- 21.5   36                                         SERVED                 SERVED                            51   19.7   36   GOOD UNOB- 21.6   35   GOOD UNOB- 21.6   36                                         SERVED                 SERVED                            52   19.8   38   BAD  UNOB- 21.6   37   BAD  UNOB- 21.6   38                                         SERVED                 SERVED                            53   20.1   38   GOOD UNOB- 21.6   38   GOOD UNOB- 21.7   39                                         SERVED                 SERVED                            54   20.2   37   BAD  UNOB- 21.7   38   BAD  OB-   21.7   37                                         SERVED                 SERVED                            COMPARISON                                                                     PULL STRENGTH (g)                                                              55   12                                                                        56   21                                                                        __________________________________________________________________________                     Pd + Mo (0.5 at %)      Pd + Cu (5 at %)                                               C                      C                               Au + Be (10 at ppm)     MODE                   MODE                            SAM- WORK-      PULL    RUP- WORK-      PULL   RUP- WORK-                      PLE  ABIL-                                                                               CHIP  STRENGTH                                                                               TURE ABIL-                                                                               CHIP  STRENGTH                                                                              TURE ABIL-                                                                               CHIP                  NO.  ITY  CRACK (g)     (n = 40)                                                                            ITY  CRACK (g)    (n = 40)                                                                            ITY  CRACK                 __________________________________________________________________________     PREFERRED EMBODIMENT                                                            1   GOOD UNOB- 20.0     8   GOOD UNOB- 22.0    7   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 2   GOOD UNOB- 20.1    26   GOOD UNOB- 22.0   23   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 3   GOOD UNOB- 20.1    29   GOOD UNOB- 22.1   27   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 4   GOOD UNOB- 20.2    32   GOOD UNOB- 22.1   31   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 5   GOOD UNOB- 20.5    34   GOOD UNOB- 22.2   33   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 6   GOOD UNOB- 20.6    31   GOOD UNOB- 22.4   35   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 7   BAD  OB-   20.7    31   BAD  OB-   22.4   32   BAD  OB-                             SERVED                  SERVED                 SERVED                 8   GOOD UNOB- 20.0     8   GOOD UNOB- 22.0    4   GOOD UNOB-                           SERVED                  SERVED                 SERVED                 9   GOOD UNOB- 20.0    25   GOOD UNOB- 22.1   23   GOOD UNOB-                           SERVED                  SERVED                 SERVED                10   GOOD UNOB- 20.1    34   GOOD UNOB- 22.1   29   GOOD UNOB-                           SERVED                  SERVED                 SERVED                11   GOOD UNOB- 20.2    37   GOOD UNOB- 22.2   29   GOOD UNOB-                           SERVED                  SERVED                 SERVED                12   GOOD UNOB- 20.6    36   GOOD UNOB- 22.3   34   GOOD UNOB-                           SERVED                  SERVED                 SERVED                13   GOOD UNOB- 20.6    33   GOOD UNOB- 22.4   33   GOOD UNOB-                           SERVED                  SERVED                 SERVED                14   BAD  OB-   20.7    32   BAD  OB-   22.4   35   BAD  OB-                             SERVED                  SERVED                 SERVED                15   GOOD UNOB- 20.0    10   GOOD UNOB- 22.0    6   GOOD UNOB-                           SERVED                  SERVED                 SERVED                16   GOOD UNOB- 20.0    26   GOOD UNOB- 22.1   22   GOOD UNOB-                           SERVED                  SERVED                 SERVED                17   GOOD UNOB- 20.2    34   GOOD UNOB- 22.1   30   GOOD UNOB-                           SERVED                  SERVED                 SERVED                18   GOOD UNOB- 20.3    37   GOOD UNOB- 22.2   38   GOOD UNOB-                           SERVED                  SERVED                 SERVED                19   GOOD UNOB- 20.5    36   GOOD UNOB- 22.2   35   GOOD UNOB-                           SERVED                  SERVED                 SERVED                20   GOOD UNOB- 20.6    33   GOOD UNOB- 22.4   34   GOOD UNOB-                           SERVED                  SERVED                 SERVED                21   BAD  OB-   20.7    35   BAD  OB-   22.4   36   BAD  OB-                             SERVED                  SERVED                 SERVED                22   GOOD UNOB- 20.0    13   GOOD UNOB- 22.0   11   GOOD UNOB-                           SERVED                  SERVED                 SERVED                23   GOOD UNOB- 20.1    24   GOOD UNOB- 22.0   23   GOOD UNOB-                           SERVED                  SERVED                 SERVED                24   GOOD UNOB- 20.2    37   GOOD UNOB- 22.0   32   GOOD UNOB-                           SERVED                  SERVED                 SERVED                25   GOOD UNOB- 20.4    38   GOOD UNOB- 22.1   35   GOOD UNOB-                           SERVED                  SERVED                 SERVED                26   BAD  UNOB- 20.4    36   BAD  UNOB- 22.1   33   BAD  UNOB-                           SERVED                  SERVED                 SERVED                27   GOOD UNOB- 20.0    10   GOOD UNOB- 22.0    9   GOOD UNOB-                           SERVED                  SERVED                 SERVED                28   GOOD UNOB- 20.0    22   GOOD UNOB- 22.0   21   GOOD UNOB-                           SERVED                  SERVED                 SERVED                29   GOOD UNOB- 20.1    30   GOOD UNOB- 22.0   27   GOOD UNOB-                           SERVED                  SERVED                 SERVED                30   GOOD UNOB- 20.3    36   GOOD UNOB- 22.1   32   GOOD UNOB-                           SERVED                  SERVED                 SERVED                31   BAD  UNOB- 20.4    38   BAD  UNOB- 22.1   35   BAD  UNOB-                           SERVED                  SERVED                 SERVED                32   GOOD UNOB- 20.0     5   GOOD UNOB- 22.0    4   GOOD UNOB-                           SERVED                  SERVED                 SERVED                33   GOOD UNOB- 20.1    25   GOOD UNOB- 22.0   22   GOOD UNOB-                           SERVED                  SERVED                 SERVED                34   GOOD UNOB- 20.4    33   GOOD UNOB- 22.1   30   GOOD UNOB-                           SERVED                  SERVED                 SERVED                35   GOOD UNOB- 20.6    37   GOOD UNOB- 22.4   35   GOOD UNOB-                           SERVED                  SERVED                 SERVED                36   BAD  OB-   20.7    35   BAD  OB-   22.4   33   BAD  OB-                             SERVED                  SERVED                 SERVED                37   GOOD UNOB- 20.1    31   GOOD UNOB- 22.1   32   GOOD UNOB-                           SERVED                  SERVED                 SERVED                38   GOOD UNOB- 20.1    39   GOOD UNOB- 22.1   38   GOOD UNOB-                           SERVED                  SERVED                 SERVED                39   GOOD UNOB- 20.3    38   GOOD UNOB- 22.1   37   GOOD UNOB-                           SERVED                  SERVED                 SERVED                40   BAD  OB-   20.7    38   BAD  OB-   22.4   35   BAD  OB-                             SERVED                  SERVED                 SERVED                41   GOOD UNOB- 20.1    25   GOOD UNOB- 22.0   24   GOOD UNOB-                           SERVED                  SERVED                 SERVED                42   GOOD UNOB- 20.0    33   GOOD UNOB- 22.0   30   GOOD UNOB-                           SERVED                  SERVED                 SERVED                43   GOOD UNOB- 20.2    38   GOOD UNOB- 22.0   35   GOOD UNOB-                           SERVED                  SERVED                 SERVED                44   GOOD UNOB- 20.2    38   GOOD UNOB- 22.0   36   GOOD UNOB-                           SERVED                  SERVED                 SERVED                45   GOOD UNOB- 20.4    37   GOOD UNOB- 22.1   37   GOOD UNOB-                           SERVED                  SERVED                 SERVED                46   BAD  UNOB- 20.3    37   BAD  UNOB- 22.1   37   BAD  UNOB-                           SERVED                  SERVED                 SERVED                47   GOOD UNOB- 20.1     9   GOOD UNOB- 22.0    6   GOOD UNOB-                           SERVED                  SERVED                 SERVED                48   GOOD UNOB- 20.1    24   GOOD UNOB- 22.0   23   GOOD UNOB-                           SERVED                  SERVED                 SERVED                49   GOOD UNOB- 20.1    28   GOOD UNOB- 22.0   27   GOOD UNOB-                           SERVED                  SERVED                 SERVED                50   BAD  UNOB- 20.3    38   BAD  UNOB- 22.1   37   BAD  UNOB-                           SERVED                  SERVED                 SERVED                51   GOOD UNOB- 20.4    37   GOOD UNOB- 22.1   37   GOOD UNOB-                           SERVED                  SERVED                 SERVED                52   BAD  UNOB- 20.4    38   BAD  UNOB- 22.1   38   BAD  UNOB-                           SERVED                  SERVED                 SERVED                53   GOOD UNOB- 20.5    39   GOOD UNOB- 22.2   38   GOOD UNOB-                           SERVED                  SERVED                 SERVED                54   BAD  OB-   20.6    39   BAD  OB-   22.3   39   BAD  OB-                             SERVED                  SERVED                 SERVED                COMPARISON                                                                     C MODE RUPTURE (n = 40)                                                        55   2                                                                         56   2                                                                         __________________________________________________________________________

As apparent from Table 4, it is appreciated that the number of times of C mode rupture in the pull test by using the samples according to the present invention is larger than that by using the samples in the comparisons departing from the composition range according to the present invention, and that the neck portion is stronger than the other portion of the base wire. Accordingly, it is understood that the above-mentioned composition range according to the present invention is optimum.

In summary, the present invention has the following advantage.

(1) The bonding wire according to the first aspect of the present invention contains high purity Au or Au alloy as a base metal and 25-10000 atppm of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au, or contains high purity Au or Au alloy as a base metal and 5-500 atppm of low boiling point element II having a boiling point lower than a melting point of the base metal and insoluble in Au, or contains high purity Au or Au alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au and low boiling point element II having a boiling point lower than the melting point of the base metal and insoluble in Au under the condition of (content of the low boiling point element I)/25+(content of the low boiling point element II)/5≧1≧(content of the low boiling point element I)/10000+(content of the low boiling point element II)/500. Further, the bonding wire according to the second aspect of the present invention contains high purity Pd or Pd alloy as a base metal and 25-10000 atppm of low boiling point element III having a boiling point lower than a melting point of the base metal and soluble in Pd, or contains high purity Pd or Pd alloy as a base metal and 5-500 atppm of low boiling point element IV having a boiling point lower than a melting point of the base metal and insoluble in Pd, or contains high purity Pd or Pd alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element III having a boiling point lower than a melting point of the base metal and soluble in Pd and low boiling point element IV having a boiling point lower than the melting point of the base metal and insoluble in Pd under the condition of (content of the low boiling point element III)/25+(content of the low boiling point element IV)/5≧1≧(content of the low boiling point element III)/10000 +(content of the low boiling point element IV)/500. With this constitution, the low boiling point element in the ball is vaporized and scattered upon formation of the ball to thereby prevent gas absorption peculiar to metal and obtain a ball satisfactory for bonding. On the other hand, the low boiling point element in the neck portion cannot be vaporized from the surface thereof, but it tends to be vaporized in the neck portion to generate a stress. Accordingly, a rupture strength of the neck portion after bonding is improved as compared with that of the base wire generating no stress. Accordingly, the strength of the neck portion can be made equal to or more than that of the base wire.

In comparison with the prior art wherein the strength of the neck portion becomes smaller than that of the base wire because of influence of heat upon formation of the ball, the bonding wire of the present invention does not generate a rupture of the neck portion, wire falling or wire sagging during the bonding work. Furthermore, in the temperature cycle life test of products, a stress generated by a repeated temperature change is dispensed to be absorbed in the whole of the base wire. Accordingly, the rupture of the neck portion where the rupture of the bonding wire is most frequently generated can be extremely reduced to thereby improve reliability.

(2) Since the rupture of the neck portion is hard to occur, the bonding wire can be greatly reduced in diameter to thereby enable reduction in bonding pitch and realize high-density mounting of LSI. 

What is claimed is:
 1. A bonding wire for a semiconductor device, said bonding wire consisting essentially of:high purity Au or Au alloy as a base metal; and 25-10000 atppm of low boiling point element I, said low boiling point element I having a boiling point lower than a melting point of the base metal, being soluble in Au, wherein said element I includes at least one member selected from the group consisting of Zn, Cd and Hg.
 2. The bonding wire according to claim 1, wherein said Au alloy contains high purity Au and at least one element selected from the group consisting of Pd, Ag, Pt, Rh, Os, Ru, Be, Ca, Ge, Y, La, Mg, Zr, Ga, In, Mo, Re, Cu and Fe.
 3. A bonding wire for a semiconductor device, said bonding wire consisting essentially of:high purity Au or Au alloy as a base metal; and 5-500 atppm of low boiling point element II, said low boiling point element II having a boiling point lower than a melting point of the base metal, being insoluble in Au, wherein said element II includes at least one member selected from the group consisting of P, S, Se, Rb and Cs.
 4. The bonding wire according to claim 3, wherein said Au alloy contains high purity Au and at least one element selected from the group consisting of Pd, Ag, Pt, Rh, Os, Ru, Be, Ca, Ge, Y, La, Mg, Zr, Ga, In, Mo, Re, Cu and Fe.
 5. A bonding wire for a semiconductor device, said bonding wire consisting essentially of:high purity Au or Au alloy as a base metal; 5-10000 atppm of low boiling point element I, said low boiling point element I having a boiling point lower than a melting point of the base metal, being soluble in Au, wherein said element I includes at least one member selected from the group consisting of Zn, Cd and Hg; and low boiling point element II, said low boiling point element II having a boiling point lower than a melting point of the base metal, being insoluble in Au, comprising at least one member selected from the group consisting of P, S, Se, Rb and Cs, and being present in an concentration so that (content of said low boiling point element I)/25+(content of said low boiling element II)/5≧1≧(content of said low boiling element I)/10000+(content of said low boiling element II)/500.
 6. The bonding wire according to claim 5, wherein said Au alloy contains high purity Au and at least one element selected from the group consisting of Pd, Ag, Pt, Rh, Os, Ru, Be, Ca, Ge, Y, La, Mg, Zr, Ga, In, Mo, Re, Cu and Fe. 